Air has basically n=1. f=v/lambda. In a different medium v->v/n, lambda->lambda/n. The frequency is unchanged. This is also sort of obvious. A thing that measures frequency is counting waves as they pass. If it's located outside of the eye it counts f waves per second. If it's inside the eye it must also count f waves per second. Otherwise, some 'waves' are getting lost somewhere. Where could they go??